Generally, the mirror polishing of semiconductor wafers such as a silicon wafer etc., lenses, and glass substrates includes rough polishing primarily intended to regulate planarity and in-plane uniformity and final polishing primarily intended to improve surface roughness and removal of scratches.
The final polishing is carried out usually by rubbing a wafer against an artificial suede made of flexible urethane foam stuck to a rotatable platen and simultaneously feeding thereon an abrasive containing a colloidal silica in an alkali-based aqueous solution (Patent Literature 1).
As the polishing pad for finishing used in final polishing, the following polishing pads have been proposed besides those described above.
A suede finishing polishing pad comprising a nap layer having a large number of long and thin holes (naps) formed with a foaming agent in the thickness direction, in polyurethane resin, and a foundation cloth for reinforcing the nap layer is proposed (Patent Literature 2).
A suede abrasive cloth for final polishing, in which surface roughness is expressed as an arithmetic average roughness (Ra) of 5 μm or less, is proposed (Patent Literature 3).
An abrasive cloth for final polishing, which is provided with a base material part and a surface layer (nap layer) formed on the base material part, wherein a polyvinyl halide or vinyl halide copolymer is contained in the surface layer, is proposed (Patent Literature 4).
Conventional polishing pads have been produced by a wet curing method. The wet curing method is a method wherein an urethane resin solution obtained by dissolving urethane resin in a water-soluble organic solvent such as dimethylformamide is applied onto a base material, then wet-solidified by treatment in water, to form a porous grain side layer, which is then washed with water and dried, followed by polishing of the grain side layer to form a surface layer (nap layer). In Patent Literature 5, for example, an abrasive cloth for finishing, having roughly spherical holes having an average particle diameter of 1 to 30 μm, is produced by the wet curing method.
Unfortunately, conventional polishing pads have a problem in which since they have a long, thin cell structure or a surface layer made of a material with low mechanical strength, their durability is low so that the planarization performance is gradually degraded or they have low polishing rate stability. Conventional polishing pads also have a problem in which cells are often clogged with polished debris (particularly pad debris) so that they have low stability of polishing characteristics and a short life.
On the other hand, the polishing pads described below are proposed for use in rough polishing.
Patent Literature 6 discloses a polishing pad for use in polishing the surface of a semiconductor device or a precursor thereof and planarizing a metal damascene structure on a semiconductor wafer, which includes a polishing layer having a E′ ratio of about 1 to 3.6 at 30° C. to 90° C., a Shore D hardness of about 40 to 70, and a tensile modulus of about 150 to 2,000 MPa at 40° C.
Patent Literature 7 discloses a mechanochemical polishing pad capable of reducing scratching of the surface of an object being polished and reducing delamination of a low-dielectric insulating film, which includes a polishing body having a storage modulus E′ (30° C.) of 120 MPa or less at 30° C. and a ratio of storage modulus E′ (30° C.) at 30° C. to storage modulus E′ (60° C.) at 60° C. (E′(30° C.)/E′(60° C.)) of 2.5 or more.
Patent Literature 8 discloses a process for reducing appearance defects such as slicing marks in a polishing layer made of a hard polyurethane foam and reducing variations in the thickness of the polishing layer so that the flatness of the polishing surface can be improved, which includes providing a polyurethane foam block with an Asker D hardness of 50 or more at ambient temperatures, controlling the surface hardness of the polyurethane foam block within the Asker hardness range of 80 to 95, and slicing the polyurethane foam block with the controlled hardness into pieces with a predetermined thickness, so that polishing sheets are formed.
Patent Literature 1: JP-A 2003-37089
Patent Literature 2: JP-A 2003-100681
Patent Literature 3: JP-A 2004-291155
Patent Literature 4: JP-A 2004-335713
Patent Literature 5: JP-A 2006-75914
Patent Literature 6: JP-A 2004-507076
Patent Literature 7: JP-A 2006-114885
Patent Literature 8: JP-A 2005-169578